Currently, world is dealing with two major hurdles towards a prosperous economy, i.e., energy crisis and environmental pollution, which challenge the development and growth of the living society. In this regard, semiconductor-oriented heterogeneous photocatalysis, electrocatalysis, and photoelectrocatalysis via fuel cell technology are green and sustainable technologies that have been receiving wide attention by the scientific community for tackling these two issues. In addition, in the present scenario, the synthesis of cost-effective and robust catalysts with high stability, corrosion resistance in any medium, and size and morphology oriented in the micro and nanoscale is a hot topic of research. Therefore, transition metal phosphides (TMPs) and their composite-based catalysts are extensively used in these fields, replacing previously used noble metals such as Pt, Au, and Ag due to scarcity, instability, and high price. Predominantly, nickel phosphide-based materials among all TMPs have proven to be excellent catalysts regarding these fields of application. The metal–phosphorus bonding, basically the Ni–P bonding present in the catalyst, and its resemblance with the [NiFe] hydrogenase makes it more stable and resistive. Several phases of nickel phosphides are available with different lattice structures. Moreover, according to their metal and phosphorus proportion, they can be divided in two categories as metal-rich nickel phosphides and phosphorus-rich nickel phosphides. With respect to this, their activities are also reflected by their elemental contents; the phosphorus-rich nickel phosphides are excellent in electrocatalytic hydrogen evolution reactions (HER), whereas metal-rich nickel phosphides are excellent in electrocatalytic oxygen evolution (OER). However, some metal-rich nickel phosphides with high P/Ni ratio, i.e., Ni5P4, show better activity towards HER because of the high P content. The Ni2P phase also plays an active role in both HER and OER, though it belongs to the metal-rich phase. This shows good catalytic behavior towards HER due to presence of well-defined exposed facets and proper proportion of Ni and P atoms. Furthermore, different types of composites are still being developed by modifying their morphology and size through different synthetic strategies with the expectation of more enhanced activity. This review gives an overview of the design and development of nanostructured nickel phosphide-based materials and their recent significant trends in electrocatalytic, photoelectrocatalytic, and photocatalytic water splitting reactions. Moreover, we present the state-of-art various nickel phosphide materials in last five years, which highlight challenges and future prospects involving some critical issues in these emerging fields that await further exploration, and opportunities in the present and future research.

Recent advances in phase, size, and morphology-oriented nanostructured nickel phosphide for overall water splitting

Metal phosphides (MPs) with unique and desirable physicochemical properties provide promising potential in practical applications, such as the catalysis, gas/humidity sensor, environmental remediation, and energy storage fields, especially for transition metal phosphides (TMPs) and MPs consisting of group IIIA and IVA metal elements. Most studies, however, on the synthesis of MP nanomaterials still face intractable challenges, encompassing the need for a more thorough understanding of the growth mechanism, strategies for large-scale synthesis of targeted high-quality MPs, and practical achievement of functional applications. This review aims at providing a comprehensive update on the controllable synthetic strategies for MPs from various metal sources. Additionally, different passivation strategies for engineering the structural and electronic properties of MP nanostructures are scrutinized. Then, we showcase the implementable applications of MP-based materials in emerging sustainable catalytic fields including electrocatalysis, photocatalysis, mild thermocatalysis, and related hybrid systems. Finally, we offer a rational perspective on future opportunities and remaining challenges for the development of MPs in the materials science and sustainable catalysis fields.

Nanostructured metal phosphides: from controllable synthesis to sustainable catalysis.

The development of highly efficient, low-cost and robust electrocatalysts for water splitting is urgently necessary to approach the energy crisis and environmental problems. In recent years, transition metal-based materials are commonly used as outstanding electrode materials for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) due to their tunable electrocatalytic activity and earth-abundance. However, they are restrained from the disadvantages of low conductivity, easy agglomeration and poor stability. Therefore, it is a good choice to integrate transition metal compounds with carbon materials to address the aforementioned issues. In this review, we focus on the recent progress of the fabrication of a variety of transition metal-based materials integrated with carbon matrices for HER, OER and overall water splitting. First, the fundamentals of the water splitting process and the introduction of parameters to determine the water splitting performance are given. Then, the advantages of the integration of transition metals and compounds with carbon for electrolytic water splitting as well as the influence of the architecture, composition, electronic structure and interface engineering of the hybrid materials on their electrocatalytic activities are discussed. The experimental and theoretical simulation are complementary to investigate the mechanism of the enhanced HER, OER and overall water splitting processes. Finally, challenges, perspectives and opportunities for developing new transition metals and carbon-based electrocatalysts in water splitting are featured.

Integrated transition metal and compounds with carbon nanomaterials for electrochemical water splitting

Carbon quantum dots (CQDs) are a class of carbon nanomaterials that have recently gained recognition as current entrants to traditional semiconductor quantum dots. CQDs have the desirable advantages of low toxicity, environmental friendliness, low cost, photostability, favorable charge transfer with enhanced electronic conductivity, and comparable easy-synthesis protocols. This review examines the advancements in CQD research and development, with a focus on their synthesis, functionalization, and energy applications. Initially, various synthesis methods are discussed briefly with pros and cons. Herein, first top-down methods including the arc-discharge technique, laser ablation technique, plasma treatment, ultrasound synthesis technique, electrochemical technique, chemical exfoliation, and combustion were discussed briefly. The later section presents bottom-up (microwave synthesis, hydrothermal synthesis, thermal pyrolysis, and metal–organic framework template-assisted approach) and waste-derived CQD synthesis methods. The next section is focused on the energy applications of CQDs including supercapacitors, lithium-ion batteries, photovoltaics, hydrogen evolution reaction and oxygen evolution reaction. Finally, challenges and future perspectives in this exciting and promising area are presented.

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Carbon Quantum Dots for Energy Applications: A Review

As a clean energy carrier, hydrogen has priority in decarbonization to build sustainable and carbon-neutral economies due to its high energy density and no pollutant emission upon combustion. Electrochemical water splitting driven by renewable electricity to produce green hydrogen with high-purity has been considered to be a promising technology. Unfortunately, the reaction of water electrolysis always requires a large excess potential, let alone the large-scale application (e.g., >500 mA cm−2 needs a cell voltage range of 1.8–2.4 V). Thus, developing cost-effective and robust transition metal electrocatalysts working at high current density is imperative and urgent for industrial electrocatalytic water splitting. In this review, the strategies and requirements for the design of self-supported electrocatalysts are summarized and discussed. Subsequently, the fundamental mechanisms of water electrolysis (OER or HER) are analyzed, and the required important evaluation parameters, relevant testing conditions and potential conversion in exploring electrocatalysts working at high current density are also introduced. Specifically, recent progress in the engineering of self-supported transition metal-based electrocatalysts for either HER or OER, as well as overall water splitting (OWS), including oxides, hydroxides, phosphides, sulfides, nitrides and alloys applied in the alkaline electrolyte at large current density condition is highlighted in detail, focusing on current advances in the nanostructure design, controllable fabrication and mechanistic understanding for enhancing the electrocatalytic performance. Finally, remaining challenges and outlooks for constructing self-supported transition metal electrocatalysts working at large current density are proposed. It is expected to give guidance and inspiration to rationally design and prepare these electrocatalysts for practical applications, and thus further promote the practical production of hydrogen via electrochemical water splitting.

Transition metal-based catalysts for electrochemical water splitting at high current density: current status and perspectives

Transition-metal phosphides (TMPs) are good electrocatalysts for the hydrogen evolution reaction (HER) due to their high catalytic efficiency and low cost. Carbon quantum dots (CQDs) deposited on top of TMPs could make them even better for the HER by increasing the surface area and the number of active sites. Here we report a method to synthesize CQD-modified manganese–nickel phosphide (CQDs/MnxNi5−xP4) for efficient and stable HER activity using inexpensive raw materials. In 0.5 M H2SO4, CQDs/MnxNi5−xP4 requires a low overpotential of only 31 mV to achieve a current density of 10 mA cm−2, as well as having a low Tafel slope of 41.0 mV dec−1, a large exchange current density of 1.753 mA cm−2, and good stability, making it better than most reported transition-metal-based catalysts. Moreover, CQDs/MnxNi5−xP4 also displays high activity and stability in alkaline solution, revealing that the ancillary role played by CQDs could be beneficial under both acidic and alkaline conditions. Based on our results, we believe that CQDs have great potential to be applied to other materials with various morphologies and structures for designing high-performance HER catalysts.

Hai-Zhi Song

Papers

The effect of carbon quantum dots on the electrocatalytic hydrogen evolution reaction of manganese–nickel phosphide nanosheets

Nickel phosphide based hydrogen producing catalyst with low overpotential and stability at high current density